1. Field of the Invention
The present invention relates to an electrode intended to be wound into a spiral, of the paste-coated type whose three-dimensional support is a conductive metal foam, for example. It relates more particularly, although not exclusively, to alkaline electrolyte cylindrical nickel-cadmium (Nixe2x80x94Cd) or nickel-hydridable metal (Nixe2x80x94MH) storage cells used for portable applications.
It further encompasses the method of manufacturing the electrode and its use in a storage cell.
2. Description of the Prior Art
A paste-coated electrode comprises a conductive support which can be a two-dimensional or plane support, such as solid or perforated strip, expanded metal, mesh or woven fabric, or a three-dimensional support such as foam or felt. The support is coated with a layer of paste containing the electrochemically active material and usually a binder.
When the storage cell is assembled, the electrode must be electrically connected to the current output terminal, generally by means of a connection fixed to the inside part of the terminal and to the conductive support of the electrode. The connection is preferably welded to the support to produce a reliable electrical contact.
In the case of cells with a spiral-wound electrode assembly it is advantageous to be able to weld the connection directly to the edge of the electrode. When the electrode support is a three-dimensional porous support with a large volume, its edge does not have the solidity needed for a connection to be welded directly to it. A number of documents therefore propose to reinforce the edge portion of the support before fixing the connection to it.
The support can be reinforced by fixing a reinforcing band in the form of a metal tape along the longitudinal edge portion.
Japanese patent application JP-62 219 462 describes a fibrous support electrode whose edge portion is reinforced by a metal band bent into a U-shape straddling and spot-welded to the support. A connecting tongue is then fixed to the band.
The use of a reinforcing band with a cut-out edge portion has been suggested. The band is compressed onto the support so that the cut-outs are inserted into the pores of the support to anchor the band to the support.
Japanese patent application JP-62 237 665 proposes to use a metal plate bent into a U-shape with a crenellated edge. The plate is compressed onto the felt support so that the upstanding portions are anchored to the support.
British patent GB-2 055 899 describes a porous support electrode. The reinforcement is a conductive plate whose edge is cut to a sawtooth shape and bent at right angles to the plate. Pressure is applied to the teeth so that they penetrate into the support. A metal deposit then consolidates the assembly.
A reinforcing band of this kind is costly to make and difficult to fit in a reliable and reproducible manner.
Another solution is to use a reinforcing band consisting of a very porous material, which can be the same as the material of the support. The reinforcing band is compressed onto the support so that the two materials interpenetrate.
European patent EP-0 658 946 describes a plane electrode whose edge portion is reinforced by compressing a plurality of layers of foam similar to that constituting the support.
According to Japanese patent application JP-57 080 672 the edge portion of a felt support is bent on itself one or more times and compressed.
All the reinforcing methods previously described stiffen the longitudinal edge portion of the electrode, making the flexing imposed by subsequently winding the electrode into a spiral very difficult.
European patent application EP-0 516 535 proposes to provide on each side of the edge portion of the electrode an expanded metal or perforated strip connection with a voids ratio exceeding 40% and fixed by compression.
This solution is difficult to use because of the high risk of short circuits. The short circuits are caused by the free metal strands at the edge of the connection, which can perforate the separator and come into contact with the opposite polarity electrode.
The object of the present invention is to propose a spiral-wound electrode with a three-dimensional support having a longitudinal edge portion which is reinforced in order to receive a connection welded to its edge, which is easier to wind into a spiral and for which there is a lower risk of short circuits than for prior art electrodes.
The invention consists in an electrode comprising a porous three-dimensional conductive support and at least one reinforcing metal band fixed along a longitudinal edge portion of the support and having a coefficient of elongation to rupture equal to at least 20% in a direction parallel to the edge portion of the support, wherein two longitudinal edge portions of the band are bent against the same face of the band, at least part of whose surface is pressed against the support.
This prevents short circuits associated with the presence of metal strands escaping from the edge portion of the band by enclosing those strands between the band and the electrode support.
To preserve the longitudinal flexibility which is indispensable for winding the electrode into a spiral, the reinforcing band must itself be highly flexible, which is possible only if the band is capable of sufficient elongation without deformation. This property is assessed by the coefficient of elongation to rupture, defined as the difference between the length at rupture and the initial length divided by the initial length of the sample, which is measured in the following manner.
A sample of the material to be characterized is inserted between the jaws of a tensile test machine. The length L0 between the jaws is 100 mm and the width of the sample is that of the band that will be used to reinforce the edge portion of the electrode. The strain is applied at a rate of 100 mm/min. The length at rupture Lr is determined from the response curve of the traction force as a function of elongation.
The coefficient of elongation to rupture y expressed as a percentage is calculated from the equation:
xcex3=100 (Lrxe2x88x92L0) /L0
The band is preferably expanded metal, mesh or perforated strip with a voids ratio of at least 40%, and preferably at least 50%. The voids ratio is defined as the ratio of the surface area of the voids to the total surface area of the band.
The band can be bent into a U-shape in its lengthwise direction so that it can straddle the edge portion of the electrode, its bent edge portions being trapped inside the U-shape. Two bands with both edge portions bent in this way can be provided on respective opposite sides of the support.
The band is preferably provided on only one side of the support, advantageously the side of the support which is on the outside during winding into a spiral. The band is then subjected to traction and stretches without deforming. If it were instead placed on the inside of the spool, it could become deformed and this deformation could cause increases in thickness compromising the spiral winding.
The support is usually a nickel foam. The band can be made of nickel, nickel-plated steel or stainless steel.
The band is advantageously welded to the support. The fixing of the band to the support is then more solid and the electrical continuity is reliable. The welding process can be a spot welding, knurled wheel welding with contiguous spots or ultrasound welding.
The invention also consists in a storage cell comprising a spiral-wound electrode of the above kind forming a spool and electrically connected to a terminal by a connection member welded to the edge of the spool and to the terminal.
The material of the connection member can be nickel, nickel-plated steel or stainless steel. The band is preferably made of the same metal as the connection member to facilitate welding.
In a first embodiment of the invention the electrode contains electrochemically active material in the form of nickel-based hydroxide.
In a second embodiment, the electrode contains a cadmium-based electrochemically active material.
In a third embodiment of the invention, the electrode contains electrochemically active material in the form of a metal alloy capable of absorbing hydrogen to form a hydride.
In a fourth embodiment of the invention the electrode contains electrochemically active material in the form of a carbon-containing material into whose structure lithium can be inserted, for example graphite, coke, carbon black and vitreous carbon.
In a fifth embodiment of the invention the electrode contains electrochemically active material in the form of an oxide of a transition metal, such as vanadium oxide, lithium-containing oxides of manganese, nickel and cobalt and lithium-containing oxides containing nickel, cobalt, manganese or aluminum, for example.
The invention will be better understood and other advantages and features of the invention will become apparent on reading the following description which is given by way of non-limiting example and with reference to the accompanying drawings.